The invention relates to a protective circuit for analog sensors, which have a power supply connection, a chassis ground connection and a sensor output connection. Such sensors are generally known and are used, for example, as pressure sensors in motor vehicles.
Specifically, in the case of motor vehicles, one of the electrical lines to the sensor may be interrupted by shocks or similar disturbing influences, either as a result of a line break, the detaching of a plug-type connection, or similar occurrences. An interruption of the ground line is particularly critical because supply voltage continues to be present on the supply voltage line and, as a result, a faulty signal can occur at the sensor output, which faulty signal cannot be differentiated from valid measured values. This can, for example, have the result that, when the ground line is interrupted, a normal operating condition (for example, a sufficient operating pressure of the braking system in the case of commercial vehicles) is indicated in the control unit, although it does not exist. This represents a considerable safety risk.
Thus, in the event of an interruption of the ground line to the sensor, the currently known analog sensors supply a measuring signal to the control unit which differs from zero. In the event of a fault, the signal voltage detected in the control unit depends considerably on the load resistance. It is therefore an object of the invention to create a protective circuit for analog sensors of the above-mentioned type which, in the event of a line interruption, particularly in the event of an interruption of the ground line, supplies a signal which can be clearly distinguished from a wanted signal.
This object is achieved by providing a protective circuit for analog sensors which are connected with a supply voltage line, a ground line and a sensor output line. One transistor, respectively, is interposed in the supply voltage line and the ground line, whose control connection is connected to one voltage divider respectively situated between the supply voltage line and the ground line. The two transistors are connected in the normal operation and are switched off at least when the ground line is interrupted. The sensor output line is connected by way of a pull-down resistor with a ground potential, which is independent of the ground line. Advantageous developments and further developments of the invention are described and claimed herein.
The basic principle of the invention consists of interposing one transistor, respectively, in the sensor in the ground line and the supply voltage line, whose control connection is coupled to voltage dividers, which are situated between the supply line and the ground line. The two transistors are switched through (on) in the normal operation and are switched off at least when the ground line is interrupted. The sensor output line is loaded/terminated by way of a pull-down resistor in the control unit and pulls the signal in the event of a fault into a secure condition (GND potential).
As a result, it is achieved that, in the event of the interruption of one of the connection lines of the analog sensor, it is ensured that the signal voltage measured, for example, in a control unit (thus the voltage at the sensor output line) becomes exactly zero, which easily demonstrates a malfunctioning of the sensor.
In the following, the invention will be explained in greater detail by means of an embodiment in connection with the drawing.